Documentation: add Device tree bindings for QCA7000

This patch adds the Device tree bindings for the
Ethernet over SPI protocol driver of the Qualcomm
QCA7000 HomePlug GreenPHY.

Signed-off-by: Stefan Wahren <stefan.wahren@i2se.com>
Signed-off-by: David S. Miller <davem@davemloft.net>